Biodiesel is a non-toxic fuel that may be used alone or blended with petroleum diesel at any ratio to create a biodiesel blend. Biodiesel has a high octane number, is essentially free of sulfur and aromatics, and is therefore a clean burning fuel, free of NOx and SOx.
Biodiesel is commonly produced by transesterification, the reaction of an alcohol with triglycerides present in animal fat or vegetable oil. Generally, such reactions are catalyzed by homogeneous catalysts such as mineral acids, metal hydroxide, metal alkoxides, and carbonates. As mineral acid catalyzed reactions are slow and therefore economically non-viable, metal hydroxides such as sodium or potassium hydroxides are more commonly used as they are relatively inexpensive and suitably effective. One disadvantage to using alkaline hydroxides or carbonates in transesterification reactions is the generation of soap as a reaction byproduct. The generation of soap compromises product yields and product quality. Glycerol (glycerine) is also produced as a byproduct, however the presence of water and soaps creates an emulsion that complicates the purification of biodiesel and the separation of glycerol from the fatty acid esters. Generally, copious amounts of acids and water are used to neutralize catalyst and remove soaps from the desirable reaction products. As a result, the increased number of steps required to obtain purified biodiesel and useable quality glycerol add tremendously to the cost of production, and also lead to a certain degree of environmental pollution.
The following equations illustrate the reactions that take place during transesterification to biodiesel by existing methods, using homogeneous catalysts.

Further attempts have been made in the prior art to replace homogeneous catalysts with solid catalysts. Such replacement of homogeneous catalysts, for example with solid metal oxides and double metal cyanides, is perceived to have the advantages of simple retrieval of catalyst, elimination of soap formation and reduction of environmental pollutants. Further, the use of solid catalysts in place of homogeneous catalysts may lead to higher-quality esters and glycerol, which are more easily separable and without added cost to refine the resulting ester (see for example U.S. Pat. No. 6,147,196 to Stern et al). In accordance with this expectation, a number of solid catalysts have now been reported in literature. These are generally based on metal oxides and double metal cyanides to effect the desired transesterification reaction shown in equation-5 below.

European patent EP-80-198-243 describes a solid, heterogeneous catalyst that is based on a mixture of iron oxide with alumina. This catalyst requires a very large catalyst to oil ratio, and extended contact time of more than 6 hours. Reaction temperatures of 280° C. to 320° C. are typically required, which results in coloration of the biodiesel and presence of impurities.
U.S. Pat. No. 5,908,946 describes catalysts prepared from mixtures of zinc oxide, and alumina zinc aluminate. While the catalyst does provide complete conversion to methyl ester, long reaction times and high temperatures are required. Moreover, the reaction is sensitive to water and free fatty acids. When free fatty acid conversion is desired, an esterification step must be carried out prior to the transesterification reaction.
U.S. Pat. No. 7,151,187 describes catalysts made by combining two or more of titanium isopropoxide, zinc oxide, alumina, and bismuth salts using nitric acid. Use of nitric acid is not desirable, as it is corrosive, toxic, and has a negative impact on the environment. Further, the use of nitric acid also impacts the basicity of the catalyst, which may affect the transesterification reaction.
It has further been shown that exchange of sodium ions in the 4 Å molecular sieves (formula: Na12[AlO2)12(SiO2)12].xH2O), with either K+ or Cs+ leads to a material with higher basicity which is essential in heterogeneous transesterification catalysis. However, testing has shown that despite enhancement of the basic sites, these ion-exchanged zeolites failed to achieve complete transformation of triglycerides to biodiesel.
A double metal cyanide catalyst-Fe2Zn3(CN)10 has also been shown to transesterify oils at relatively lower temperatures. However, the slow pace of reaction leads to extended reaction time and requires excessive catalyst and reactor volume.
A suitable heterogeneous catalyst and method for complete transformation of triglycerides to biodiesel and for conversion of free fatty acids to corresponding esters has not been described to date. Further, such reactions do not appear to be currently possible under mild temperature and pressure conditions, while minimizing reaction time and product purification steps.